Fluorescent tubes and incandescent light bulbs are current mainstream light sources for illumination; however, compared to a fluorescent tube or the like, those using LEDs (light-emitting diodes) for light source have less electrical power consumption, longer life span, and are safe as they are not hot to the touch, moreover, since they do not contain harmful substances such as mercury and the like, they have excellent environmental aspects, such that they are anticipated to become a mainstream light source for illumination in the near future, and a variety of research and development are being undertaken.
White light can be achieved only when lights of a variety of colors (wavelengths) are mixed. The color of an LED light, since it depends on the difference (band gap) when a high energy electron drops to a low energy position and this band gap is specific to the semiconductor crystal used in an LED chip, is fundamentally a monochromatic light such as red, green and blue, and a white light cannot be obtained from only one LED.
As a method for obtaining white light using an LED, a method has been proposed, whereby an LED serving as a light emitter and phosphor are combined, light emitted from the LED is absorbed by the phosphor, the absorbed light is wavelength-converted by this phosphor into a light having a different wavelength, white light is obtained by diffused color mixing of the light emission from the LED and the light emission from the phosphor, or, by only the light emission that has been wavelength-converted by the phosphor.
For instance, a white light-emitting device is known, in which a light-emitting diode chip (GaN), which emits blue light, and a YAG series phosphor (Y3Al5O12:Ce or the like), which emits light in yellow, are combined, a portion of the blue light radiated by the light-emitting diode chip (GaN) is left to be transmitted by the YAG series phosphor layer and the remainder is left to hit the YAG series phosphor and emit a yellow light, and these lights of two colors being color-mixed to create a pseudo-white light (refer to Patent Reference 1).
The white light-emitting device of such a constitution obtains pseudo-white light constituted by blue and yellow in terms of color components. Therefore, it has poor color rendering properties, that is to say, the property of the color of an object to be seen identically to when illuminated with natural light, and is said to be ill-suited of general illumination.
In Patent Reference 2, a light-emitting device comprising a blue LED, a green phosphor composition and a red phosphor composition is disclosed. This light-emitting device lets the light-emission (blue light) from the blue LED be received by each of the green phosphor composition and red phosphor composition, which respectively emit light with long wavelengths (green light and red light), to obtain white light by diffused color mixing of the blue light from the blue LED and the green light and red light from the phosphor compositions.
Since this light-emitting device uses a plurality of phosphors (compositions), color adjustment is difficult due to the interaction between the phosphors. In particular, as blending to adjust the contents in phosphors emitting light for each color, the mixing method and the like, is difficult, it bears the issue that variations occur in the quality of the product.
If white light could be achieved with a single phosphor, it would be useful since there would be no need to blend various phosphors, allowing the variations in the quality of the products to be suppressed. However, the few are known in prior art as white phosphors comprising a single material, the extent being that, for instance in Patent Reference 3, as a phosphor emitting red or white light by an excitation light in the near ultraviolet to visible region, a white light-emitting device is disclosed, which is a silicate of the M2SiO4 type containing Ba and Ca activated with Eu and Mn, allowing a white light-emitting to be obtained by controlling the molar ratio of Mn to a small positive value.
Patent Reference 1: JP Patent No. 3503139
Patent Reference 2: Japanese Patent Application Laid-open No. 2005-20010
Patent Reference 3: Japanese Patent Application Laid-open No. 2005-226069, in particular [0017]